Odor detection is effected through olfactory receptors which are located in neurons in the olfactory epithelium in the nasal cavity. The signals from these neurons pass on to the glomeruli in the olfactory bulb and onto the higher center of the brain for further interpretation. Each receptor neuron expresses a single class of olfactory receptor, and olfactory receptor neurons of such a single type are distributed across the olfactory epithelium. The output fibers from these scattered neurons converge together on a single glomerulus in the olfactory bulb. Thus the signals from olfactory neurons coding for similar molecular properties/moieties carrying the same odor informational content will tend to converge on the same glomeruli in the olfactory bulb. A single odorant molecule will generally excite more than one class of olfactory neuron, and the pattern of excitation will be reproducible and characteristic of that molecule.
In this process the features of the odorant molecule are first fragmented and detected by the odor receptors. Then similar features of different odor molecules reinforce each other at the different odor receptors, and at the olfactory bulb level. The whole is then re-integrated to provide the odor perception, which can be as simple as a single percept. In this way the many odorous molecules emanating from a single flower can excite multiple neurons, whose signals recombine to produce a single olfactory experience which the observer can recognise as typical of the particular flower. A different flower may emit many of the same materials but the differences in levels and composition will be re-integrated to yield a different sensory percept that can be recognised as coming from the different flower.
This combinatorial approach has been proposed previously, but the detailed processes involved are yet far from understood. The complexity of the combinatorial mechanisms has been a recurring feature of olfactory research. Early studies of odor mixtures sought to chart and classify the sensory phenomena when odors were mixed, and developed terms to describe the observed changes in total intensity that were observed. These studies were limited to binary mixes due to the complexity of the phenomena involved.
Progress has proved equally tricky at a biological level. It has been observed that single olfactory neurons simultaneously integrated several chemical signals. However researchers stress that complex interactions occur between components, and that the responses of olfactory neurons are not simply predictable from the responses of their components. They found that the events that occurred at the receptor neurons themselves, without the contribution of later events at the olfactory bulb, could be linked to changes in perceived odor, e.g. due to one odorant dominating or even masking the effect of another. A natural odor would induce a multi-chemical integration at the olfactory receptor neuron which might be equivalent to a shift in their odor coding properties, such that they may play a major part in perception process as a whole.
Thus the issues underlying the challenge for researchers trying to understand odors are becoming clearer while the complexity and non-linearity of the observed phenomena is making even reliable classification difficult.
In nature it is common for the odor experience to arise from a complex mixture of odor molecules and for this mixture to be perceived as a single percept. This circumstance can be observed in animals and insects where olfactory signals can drive critical behaviours. For example, a moth can identify a flower which emits more than 60 materials of which 9 are detected by the olfactory system. These have been shown to behave as a single percept capable of driving flower-foraging behaviour. The encoding is organised through a population of glomerular coding units which are thought to combine the different features of the molecular stimulants into the singular percept (via a mechanism as yet unknown).
In human studies the detailed outcome of such odor mixing has been variable and unpredictable though some broad categories of response are regularly observed.
The convergent nature of processes occurring at the higher centres of olfactory processing necessarily means that odor mixtures are not always simple combinations of their components. This being said it is often possible for humans to perceive a complex odor mixture as a single whole, while also being able to decompose the experience into sensory sub-units. For example, when a malodor and perfume are mixed it is often possible to compartmentalise the experience such that the relative contributions of each odor type to the overall odor can be judged. So there exists a paradox: that the mix may be perceived as a single perceptual experience, while that experience may be subdivided on introspection.
The outcome of introspection may not reflect the relative intensities of the component stimuli, or even their odor character. Nevertheless the process can be sufficiently reproducible that it can be used to design new products which deliver useful benefits, e.g. deodorant perfumes.
In such masking scenarios it is usual for one odor to be employed to reduce the perception of a second, less-desirable odor. This is a common practice and routes to optimise the process have been developed. Examples of synergistic interactions between odors are extremely rare by comparison.
In a compilation study based on the results from 520 binary mixtures, the most likely outcome of odor mixing at levels above threshold was that the total intensity of the mix was below the sum of the component intensities, and below that which would be expected from auto-addition following Stevens' Law. Intensity of a single material tends to increase as a logarithmic function of its concentration (Stevens' Law), so the first of these findings is not unexpected, however the second finding is more surprising. It was also found that one of the two components reduced the intensity of the other, more than occurred the other way round. They also found that adding a third, fourth, or fifth iso-intense component did not lead to any increase in overall intensity. This indicated strong compression mechanisms in play.
As noted above, synergistic effects were found to be infrequent. When found, they were thought to be associated with ‘synthetic phenomena’, where a new different odor quality is created when mixing the two components. Some odor was perceived when mixing sub-threshold levels of odorants but it was not possible to rationalise the observations. It was concluded that any study of these effects would require both intensity and odor character to be measured simultaneously.
Synergy has been described as a higher level of sensory impact than one would expect based on the impacts of the unmixed components. One example is adding a sub-threshold amount of one odorant causing a small but measurable increase in the perceived intensity of another (beverage) odor or in the perceived sweetness of supra-threshold sucrose. It has been thought that the addition of small amounts of one material can occasionally lead to significant increases in the intensity of an aroma or flavour. However, these examples may not be considered definitive examples of synergy unless the sub-threshold stimuli had no odor themselves. Given the statistical nature of a threshold measure (e.g. the level at which 50% of subjects can detect its presence, and therefore 50% of subjects cannot) the added materials will have been supra-threshold for many of the subjects.
With such issues in mind, the first clear, unambiguous demonstration of synergy in odor detection in humans was shown. The materials were maple lactone mixed with the volatile carboxylic acids, acetic acid and butyric acid. Generally at detection threshold for binary mixtures, the threshold concentration of an individual component tended to be lower than the threshold of the component smelled alone, a phenomenon referred to as Agonism.
Researchers extended their studies to 3-component mixtures, but no universal theme emerged. They concluded that the rules for mixture interactions were such that each mixture must be treated separately and empirically.
In another supra-threshold study, binary mixes of a fruity and a woody odor, using ortho-nasal and retro-nasal stimulation were examined. The fruity intensity could be increased or decreased in mixtures depending on the level of the woody component. Synergy was reported based on eeg measures, where an enlarged N1 peak amplitude was found in some mixes. Other mixes, smelled retro-nasally, showed increased P2 amplitudes during eeg scans. These results may be evidence of both sensory and cognitive processes in play simultaneously during odor perception.
A study of alkyl sulphides and thiols led to the conclusion that the mixing of such materials with similar chemical structure could be characterised by an averaging effect over all components.
Binary mixes of L-carvone (caraway odor) and eugenol (clove odor) were presented at one nostril as a physical mixture versus each odorant presented separately at separate nostrils (dichorhinic mixing). Psychophysical and eeg responses were recorded. The dichorhinic mixtures were perceived as stronger then the physical mixes. The perceived odor character also differed between the two assessment methods. The eeg responses for the dichorhinic mixes showed differences for the P1 & N1 (more sensory) peaks. Taken together all the results show that significant Left-Right hemispheric interactions take place at the higher centers of the brain (or at least, post-glomeruli), and that the peripheral level is a site of significant interaction too.
In a later publication, it was shown that mixture quality (character) is not tied to any particular single component, indicating that we perceive an odor mixture more or less synthetically as a single percept. In his study the odor and its pleasantness of a mixture was generally intermediate between that of each of the individual components.
WO2002049600, which is incorporated by reference herein in its entirety, discloses perfume compositions with specific components to promote relaxed mood states.
The present invention seeks to address at least some of the issues described above. Specifically to identify groups of odor ingredients that can be used to create synergistic odor or perfume compositions and the resulting perfume compositions therefrom.